407 lines
13 KiB
Python
407 lines
13 KiB
Python
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'''Functions returning normal forms of matrices'''
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from collections import defaultdict
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from .domainmatrix import DomainMatrix
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from .exceptions import DMDomainError, DMShapeError
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from sympy.ntheory.modular import symmetric_residue
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from sympy.polys.domains import QQ, ZZ
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# TODO (future work):
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# There are faster algorithms for Smith and Hermite normal forms, which
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# we should implement. See e.g. the Kannan-Bachem algorithm:
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# <https://www.researchgate.net/publication/220617516_Polynomial_Algorithms_for_Computing_the_Smith_and_Hermite_Normal_Forms_of_an_Integer_Matrix>
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def smith_normal_form(m):
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'''
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Return the Smith Normal Form of a matrix `m` over the ring `domain`.
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This will only work if the ring is a principal ideal domain.
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Examples
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========
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>>> from sympy import ZZ
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>>> from sympy.polys.matrices import DomainMatrix
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>>> from sympy.polys.matrices.normalforms import smith_normal_form
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>>> m = DomainMatrix([[ZZ(12), ZZ(6), ZZ(4)],
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... [ZZ(3), ZZ(9), ZZ(6)],
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... [ZZ(2), ZZ(16), ZZ(14)]], (3, 3), ZZ)
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>>> print(smith_normal_form(m).to_Matrix())
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Matrix([[1, 0, 0], [0, 10, 0], [0, 0, -30]])
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'''
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invs = invariant_factors(m)
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smf = DomainMatrix.diag(invs, m.domain, m.shape)
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return smf
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def add_columns(m, i, j, a, b, c, d):
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# replace m[:, i] by a*m[:, i] + b*m[:, j]
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# and m[:, j] by c*m[:, i] + d*m[:, j]
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for k in range(len(m)):
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e = m[k][i]
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m[k][i] = a*e + b*m[k][j]
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m[k][j] = c*e + d*m[k][j]
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def invariant_factors(m):
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'''
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Return the tuple of abelian invariants for a matrix `m`
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(as in the Smith-Normal form)
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References
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==========
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[1] https://en.wikipedia.org/wiki/Smith_normal_form#Algorithm
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[2] https://web.archive.org/web/20200331143852/https://sierra.nmsu.edu/morandi/notes/SmithNormalForm.pdf
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'''
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domain = m.domain
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if not domain.is_PID:
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msg = "The matrix entries must be over a principal ideal domain"
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raise ValueError(msg)
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if 0 in m.shape:
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return ()
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rows, cols = shape = m.shape
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m = list(m.to_dense().rep)
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def add_rows(m, i, j, a, b, c, d):
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# replace m[i, :] by a*m[i, :] + b*m[j, :]
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# and m[j, :] by c*m[i, :] + d*m[j, :]
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for k in range(cols):
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e = m[i][k]
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m[i][k] = a*e + b*m[j][k]
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m[j][k] = c*e + d*m[j][k]
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def clear_column(m):
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# make m[1:, 0] zero by row and column operations
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if m[0][0] == 0:
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return m # pragma: nocover
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pivot = m[0][0]
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for j in range(1, rows):
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if m[j][0] == 0:
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continue
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d, r = domain.div(m[j][0], pivot)
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if r == 0:
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add_rows(m, 0, j, 1, 0, -d, 1)
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else:
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a, b, g = domain.gcdex(pivot, m[j][0])
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d_0 = domain.div(m[j][0], g)[0]
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d_j = domain.div(pivot, g)[0]
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add_rows(m, 0, j, a, b, d_0, -d_j)
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pivot = g
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return m
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def clear_row(m):
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# make m[0, 1:] zero by row and column operations
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if m[0][0] == 0:
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return m # pragma: nocover
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pivot = m[0][0]
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for j in range(1, cols):
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if m[0][j] == 0:
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continue
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d, r = domain.div(m[0][j], pivot)
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if r == 0:
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add_columns(m, 0, j, 1, 0, -d, 1)
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else:
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a, b, g = domain.gcdex(pivot, m[0][j])
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d_0 = domain.div(m[0][j], g)[0]
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d_j = domain.div(pivot, g)[0]
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add_columns(m, 0, j, a, b, d_0, -d_j)
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pivot = g
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return m
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# permute the rows and columns until m[0,0] is non-zero if possible
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ind = [i for i in range(rows) if m[i][0] != 0]
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if ind and ind[0] != 0:
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m[0], m[ind[0]] = m[ind[0]], m[0]
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else:
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ind = [j for j in range(cols) if m[0][j] != 0]
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if ind and ind[0] != 0:
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for row in m:
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row[0], row[ind[0]] = row[ind[0]], row[0]
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# make the first row and column except m[0,0] zero
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while (any(m[0][i] != 0 for i in range(1,cols)) or
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any(m[i][0] != 0 for i in range(1,rows))):
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m = clear_column(m)
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m = clear_row(m)
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if 1 in shape:
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invs = ()
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else:
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lower_right = DomainMatrix([r[1:] for r in m[1:]], (rows-1, cols-1), domain)
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invs = invariant_factors(lower_right)
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if m[0][0]:
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result = [m[0][0]]
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result.extend(invs)
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# in case m[0] doesn't divide the invariants of the rest of the matrix
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for i in range(len(result)-1):
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if result[i] and domain.div(result[i+1], result[i])[1] != 0:
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g = domain.gcd(result[i+1], result[i])
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result[i+1] = domain.div(result[i], g)[0]*result[i+1]
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result[i] = g
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else:
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break
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else:
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result = invs + (m[0][0],)
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return tuple(result)
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def _gcdex(a, b):
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r"""
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This supports the functions that compute Hermite Normal Form.
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Explanation
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===========
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Let x, y be the coefficients returned by the extended Euclidean
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Algorithm, so that x*a + y*b = g. In the algorithms for computing HNF,
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it is critical that x, y not only satisfy the condition of being small
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in magnitude -- namely that |x| <= |b|/g, |y| <- |a|/g -- but also that
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y == 0 when a | b.
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"""
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x, y, g = ZZ.gcdex(a, b)
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if a != 0 and b % a == 0:
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y = 0
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x = -1 if a < 0 else 1
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return x, y, g
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def _hermite_normal_form(A):
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r"""
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Compute the Hermite Normal Form of DomainMatrix *A* over :ref:`ZZ`.
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Parameters
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==========
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A : :py:class:`~.DomainMatrix` over domain :ref:`ZZ`.
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Returns
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=======
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:py:class:`~.DomainMatrix`
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The HNF of matrix *A*.
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Raises
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======
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DMDomainError
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If the domain of the matrix is not :ref:`ZZ`.
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References
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==========
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.. [1] Cohen, H. *A Course in Computational Algebraic Number Theory.*
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(See Algorithm 2.4.5.)
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"""
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if not A.domain.is_ZZ:
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raise DMDomainError('Matrix must be over domain ZZ.')
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# We work one row at a time, starting from the bottom row, and working our
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# way up.
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m, n = A.shape
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A = A.to_dense().rep.copy()
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# Our goal is to put pivot entries in the rightmost columns.
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# Invariant: Before processing each row, k should be the index of the
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# leftmost column in which we have so far put a pivot.
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k = n
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for i in range(m - 1, -1, -1):
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if k == 0:
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# This case can arise when n < m and we've already found n pivots.
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# We don't need to consider any more rows, because this is already
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# the maximum possible number of pivots.
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break
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k -= 1
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# k now points to the column in which we want to put a pivot.
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# We want zeros in all entries to the left of the pivot column.
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for j in range(k - 1, -1, -1):
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if A[i][j] != 0:
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# Replace cols j, k by lin combs of these cols such that, in row i,
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# col j has 0, while col k has the gcd of their row i entries. Note
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# that this ensures a nonzero entry in col k.
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u, v, d = _gcdex(A[i][k], A[i][j])
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r, s = A[i][k] // d, A[i][j] // d
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add_columns(A, k, j, u, v, -s, r)
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b = A[i][k]
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# Do not want the pivot entry to be negative.
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if b < 0:
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add_columns(A, k, k, -1, 0, -1, 0)
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b = -b
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# The pivot entry will be 0 iff the row was 0 from the pivot col all the
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# way to the left. In this case, we are still working on the same pivot
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# col for the next row. Therefore:
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if b == 0:
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k += 1
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# If the pivot entry is nonzero, then we want to reduce all entries to its
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# right in the sense of the division algorithm, i.e. make them all remainders
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# w.r.t. the pivot as divisor.
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else:
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for j in range(k + 1, n):
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q = A[i][j] // b
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add_columns(A, j, k, 1, -q, 0, 1)
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# Finally, the HNF consists of those columns of A in which we succeeded in making
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# a nonzero pivot.
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return DomainMatrix.from_rep(A)[:, k:]
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def _hermite_normal_form_modulo_D(A, D):
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r"""
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Perform the mod *D* Hermite Normal Form reduction algorithm on
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:py:class:`~.DomainMatrix` *A*.
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Explanation
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===========
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If *A* is an $m \times n$ matrix of rank $m$, having Hermite Normal Form
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$W$, and if *D* is any positive integer known in advance to be a multiple
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of $\det(W)$, then the HNF of *A* can be computed by an algorithm that
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works mod *D* in order to prevent coefficient explosion.
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Parameters
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==========
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A : :py:class:`~.DomainMatrix` over :ref:`ZZ`
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$m \times n$ matrix, having rank $m$.
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D : :ref:`ZZ`
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Positive integer, known to be a multiple of the determinant of the
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HNF of *A*.
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Returns
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=======
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:py:class:`~.DomainMatrix`
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The HNF of matrix *A*.
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Raises
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======
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DMDomainError
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If the domain of the matrix is not :ref:`ZZ`, or
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if *D* is given but is not in :ref:`ZZ`.
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DMShapeError
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If the matrix has more rows than columns.
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References
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==========
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.. [1] Cohen, H. *A Course in Computational Algebraic Number Theory.*
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(See Algorithm 2.4.8.)
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"""
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if not A.domain.is_ZZ:
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raise DMDomainError('Matrix must be over domain ZZ.')
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if not ZZ.of_type(D) or D < 1:
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raise DMDomainError('Modulus D must be positive element of domain ZZ.')
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def add_columns_mod_R(m, R, i, j, a, b, c, d):
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# replace m[:, i] by (a*m[:, i] + b*m[:, j]) % R
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# and m[:, j] by (c*m[:, i] + d*m[:, j]) % R
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for k in range(len(m)):
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e = m[k][i]
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m[k][i] = symmetric_residue((a * e + b * m[k][j]) % R, R)
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m[k][j] = symmetric_residue((c * e + d * m[k][j]) % R, R)
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W = defaultdict(dict)
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m, n = A.shape
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if n < m:
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raise DMShapeError('Matrix must have at least as many columns as rows.')
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A = A.to_dense().rep.copy()
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k = n
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R = D
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for i in range(m - 1, -1, -1):
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k -= 1
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for j in range(k - 1, -1, -1):
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if A[i][j] != 0:
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u, v, d = _gcdex(A[i][k], A[i][j])
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r, s = A[i][k] // d, A[i][j] // d
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add_columns_mod_R(A, R, k, j, u, v, -s, r)
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b = A[i][k]
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if b == 0:
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A[i][k] = b = R
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u, v, d = _gcdex(b, R)
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for ii in range(m):
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W[ii][i] = u*A[ii][k] % R
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if W[i][i] == 0:
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W[i][i] = R
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for j in range(i + 1, m):
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q = W[i][j] // W[i][i]
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add_columns(W, j, i, 1, -q, 0, 1)
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R //= d
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return DomainMatrix(W, (m, m), ZZ).to_dense()
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def hermite_normal_form(A, *, D=None, check_rank=False):
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r"""
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Compute the Hermite Normal Form of :py:class:`~.DomainMatrix` *A* over
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:ref:`ZZ`.
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Examples
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========
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>>> from sympy import ZZ
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>>> from sympy.polys.matrices import DomainMatrix
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>>> from sympy.polys.matrices.normalforms import hermite_normal_form
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>>> m = DomainMatrix([[ZZ(12), ZZ(6), ZZ(4)],
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|
... [ZZ(3), ZZ(9), ZZ(6)],
|
||
|
|
... [ZZ(2), ZZ(16), ZZ(14)]], (3, 3), ZZ)
|
||
|
|
>>> print(hermite_normal_form(m).to_Matrix())
|
||
|
|
Matrix([[10, 0, 2], [0, 15, 3], [0, 0, 2]])
|
||
|
|
|
||
|
|
Parameters
|
||
|
|
==========
|
||
|
|
|
||
|
|
A : $m \times n$ ``DomainMatrix`` over :ref:`ZZ`.
|
||
|
|
|
||
|
|
D : :ref:`ZZ`, optional
|
||
|
|
Let $W$ be the HNF of *A*. If known in advance, a positive integer *D*
|
||
|
|
being any multiple of $\det(W)$ may be provided. In this case, if *A*
|
||
|
|
also has rank $m$, then we may use an alternative algorithm that works
|
||
|
|
mod *D* in order to prevent coefficient explosion.
|
||
|
|
|
||
|
|
check_rank : boolean, optional (default=False)
|
||
|
|
The basic assumption is that, if you pass a value for *D*, then
|
||
|
|
you already believe that *A* has rank $m$, so we do not waste time
|
||
|
|
checking it for you. If you do want this to be checked (and the
|
||
|
|
ordinary, non-modulo *D* algorithm to be used if the check fails), then
|
||
|
|
set *check_rank* to ``True``.
|
||
|
|
|
||
|
|
Returns
|
||
|
|
=======
|
||
|
|
|
||
|
|
:py:class:`~.DomainMatrix`
|
||
|
|
The HNF of matrix *A*.
|
||
|
|
|
||
|
|
Raises
|
||
|
|
======
|
||
|
|
|
||
|
|
DMDomainError
|
||
|
|
If the domain of the matrix is not :ref:`ZZ`, or
|
||
|
|
if *D* is given but is not in :ref:`ZZ`.
|
||
|
|
|
||
|
|
DMShapeError
|
||
|
|
If the mod *D* algorithm is used but the matrix has more rows than
|
||
|
|
columns.
|
||
|
|
|
||
|
|
References
|
||
|
|
==========
|
||
|
|
|
||
|
|
.. [1] Cohen, H. *A Course in Computational Algebraic Number Theory.*
|
||
|
|
(See Algorithms 2.4.5 and 2.4.8.)
|
||
|
|
|
||
|
|
"""
|
||
|
|
if not A.domain.is_ZZ:
|
||
|
|
raise DMDomainError('Matrix must be over domain ZZ.')
|
||
|
|
if D is not None and (not check_rank or A.convert_to(QQ).rank() == A.shape[0]):
|
||
|
|
return _hermite_normal_form_modulo_D(A, D)
|
||
|
|
else:
|
||
|
|
return _hermite_normal_form(A)
|